Re: [PATCH 5/6] slab: validate slab before using it in alloc_single_from_partial()

Harry Yoo
2025-09-12T10:48:58Z

On Thu, Sep 11, 2025 at 07:02:38PM +0200, Vlastimil Babka wrote:
> We touch slab->freelist and slab->inuse before checking the slab pointer
> is actually sane. Do that validation first, which will be safer. We can
> thus also remove the check from alloc_debug_processing().
>
> This adds a new "s->flags & SLAB_CONSISTENCY_CHECKS" test but
> alloc_single_from_partial() is only called for caches with debugging
> enabled so it's acceptable.
>
> In alloc_single_from_new_slab() we just created the struct slab and call
> alloc_debug_processing() to mainly set up redzones, tracking etc, while
> not really expecting the consistency checks to fail. Thus don't validate
> it there.
>
> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
> ---
> mm/slub.c | 17 ++++++++---------
> 1 file changed, 8 insertions(+), 9 deletions(-)
>
> diff --git a/mm/slub.c b/mm/slub.c
> index 909c71372a2f542b6e0d67c12ea683133b246b66..93df6e82af37c798c3fa5574c9d825f0f4a83013 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -1651,11 +1651,6 @@ static noinline bool alloc_debug_processing(struct kmem_cache *s,
> struct slab *slab, void *object, int orig_size)
> {
> if (s->flags & SLAB_CONSISTENCY_CHECKS) {
> - if (!validate_slab_ptr(slab)) {
> - slab_err(s, slab, "Not a valid slab page");
> - return false;
> - }
> -
> if (!alloc_consistency_checks(s, slab, object))
> goto bad;
> }
> @@ -2825,15 +2820,19 @@ static void *alloc_single_from_partial(struct kmem_cache *s,
>
> lockdep_assert_held(&n->list_lock);
>
> + if (s->flags & SLAB_CONSISTENCY_CHECKS) {
> + if (!validate_slab_ptr(slab)) {
> + slab_err(s, slab, "Not a valid slab page");
> + return NULL;
> + }
> + }
> +
> object = slab->freelist;
> slab->freelist = get_freepointer(s, object);
> slab->inuse++;
>
> - if (!alloc_debug_processing(s, slab, object, orig_size)) {
> - if (validate_slab_ptr(slab))
> - remove_partial(n, slab);
> + if (!alloc_debug_processing(s, slab, object, orig_size))
> return NULL;

Is it intentional to not remove the slab from the partial list
when alloc_debug_processing() returns false?

> - }
>
> if (slab->inuse == slab->objects) {
> remove_partial(n, slab);
>
> --
> 2.51.0

--
Cheers,
Harry / Hyeonggon